Merge pull request #197 from dsharlet-intel/master

Using a single global context for OpenCL
Added math function definitions to OpenCL C codegen
Merge branch 'master' of https://github.com/halide/Halide
Added a tutorial on reductions.

Also updated tutorial todos.
Text changes in latest tutorial.
Remove "x86 android is untested" warning.

It's running fine on a Galaxy Tab 3.

So I've moved the warning to the (not-yet-in-existence)
x86-64-on-android branch :-)
Merge branch 'master' of https://github.com/halide/Halide